This invention relates generally to radar signature and, in particular, to a method of reducing radar cross-section and/or echo length.
The use of radar is now widespread, for both commercial and military uses. In military applications, particularly during times of war, it may be essential that a vehicle such as an aircraft go undetected.
The radar signature or xe2x80x9ccross-sectionxe2x80x9d of an object is a measure of how much radar energy is reflected back or xe2x80x9creturnedxe2x80x9d to a source or system searching for the object. The greater the signature, the easier it is to detect, track and potentially direct weapon systems against that object.
Radar cross section is also a function of the direction from which an object is xe2x80x9cviewed.xe2x80x9d With regard to aircraft, reduced aircraft radar cross-section is most important when viewed from the front, or in the xe2x80x9cfrontal sector.xe2x80x9d Radar cross section is also typically increased in the presence of externally supported appendages such as weapons, which are typically mounted on pylons or against the body of the aircraft.
There are several techniques that may be employed to minimize the radar cross section. Broadly, one class of techniques is used to design aircraft having an inherently low radar signature, whereas other approaches seek to modify existing aircraft to achieve this same purpose. Of course, both broad principles may be applied to the same structure.
As discussed in U.S. Pat. No. 5,717,397, radar cross-section may be minimized using any combination of the following:
1. Shaping the exterior of the aircraft or external features, including leading/trailing edges, gaps, and seams, such that radar energy is reflected away from potential enemy radars;
2. Aligning leading and trailing edges, gaps and seams at a minimum number of similar angles (especially in the top or xe2x80x9cplanxe2x80x9d view of the aircraft), such that the radar returns from these various features are concentrated into fewer angles or sectors.
3. Concealing or hiding highly radar reflective aircraft components from the xe2x80x9cviewxe2x80x9d of potential enemy radars; and
4. Utilizing materials and coatings in the construction of aircraft components that absorb or diffuse radar energy.
Whether designing a craft for a low radar signature in the first place, or modifying existing craft to achieve a reduced cross. section, the problem is complex and often mathematically intensive. Cross-section optimization using manual and empirical methods is labor intensive and, although computer methods may be employed to find local minima, global optimization is often elusive. Current automated techniques use Z-matrix calculations, often requiring numerous iterations to achieve dubious results. Although such techniques have improved in recent years, existing methods often still require mechanisms to avoid stagnating in local minima.
Broadly, this invention applies variational calculus principles directly to the radiation integral to minimize radar cross-section and/or echo length. The radiation integral, which is well known to those of skill in antenna design and other disciplines, may be used to determine the electromagnetic field scattering of a body given the surface current. In the preferred embodiment of this invention, the radiation integral is minimized through the solution to a differential equation generated by Euler""s calculus of variations (CoV) equation. When used in conjunction with a minimizing sequence, the analysis affords a broad search of all possible coefficient values to ultimately arrive at. global minima.
Compared to existing techniques, the approach locates local extrema quickly and accurately using fewer impedance matrix calculations. The method is applicable to a wide variety of situations, including the design of stealth platforms. A thorough analysis of the applicable design equations is disclosed, which indicate that optimization over a wide band of frequencies and angles is possible. Although the examples presented are in two dimensions, the procedure is readily extensible to three dimensions.